The American, Will Nguyen, was visiting Ho Chi Minh City ahead of his graduation this summer from a master’s program at the University of Singapore, according to a statement from his family and friends.

Mr. Nguyen, 32, a Houston native who graduated from Yale, took part in protests on Sunday. He was “beaten over the head and dragged into the back of a police truck,” after the authorities moved to quash the demonstrations that day, according to the statement.

A video from the protests shows Mr. Nguyen, with blood smeared across his face, being dragged by a group of men. He is later shown standing in the bed of a pickup truck topped with emergency lights.

He was taken to a police station, but his current whereabouts and physical condition are not known, the statement said.

Mr. Nguyen’s family fled South Vietnam after the war that led to its collapse, he wrote in a recent piecefor the website New Naratif that discussed the conflict and the country’s history of divisions between North and South.

“He is a proud Vietnamese-American, and passionate about his studies, specifically Southeast Asian studies, in which he majored,” his sister, Victoria Nguyen, said by email.

Pope Thrower, spokesman for the United States Embassy in Hanoi, said the embassy was “aware of media reports that a U.S. citizen was arrested in Vietnam.”

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Will Nguyen, 32, of Houston, has been studying at the University of Singapore.CreditNguyen family

“When a U.S. citizen is detained overseas, the U.S. Department of State works to provide all appropriate consular assistance,” he added. “Due to privacy considerations, we have no further comment.”

From Ho Chi Minh City, commonly called Saigon, Mr. Nguyen posted a series of tweets documenting the protests on Sunday, with crowds of people marching down city streets.

“I can’t stress how enormous of an achievement this is for the #Vietnamese people,” he wrote. “The communist government is allowing people to assemble peacefully and the people are exercising their civic duty to protest injustice.”

One image he posted shows a protester who was struck by police officers lying on the street while another person helps him. Another shows a protester holding a sign that reads, “No Chinese Land Lease Even 1 Day.”

Mr. Nguyen’s family has not been able to reach him, but his hosts at an Airbnb rental did reportedly speak with him shortly after his detention. Police officers showed up at the apartment two days later to confiscate his laptop, passport, credit cards and a change of clothes, his family said.

In addition to the special economic zones, protesters said they were concerned about a proposed cybersecurity law. The state-controlled news media in Vietnam reported that 102 people were arrested Sunday in the southeastern province of Binh Thuan, where thousands of protesters blocked a highway and later set fire to public buildings. Protests were also reported in Hanoi, the capital.

The proposed special economic zones would give leases of up to 99 years to foreign investors in three areas that would have fewer administrative restrictions than the rest of the country. The proposal has stirred fear that it would undermine national security by giving China control over parts of Vietnamese territory.

Vietnam and China have overlapping claims in the South China Sea, and Chinese efforts to extend control have set off protests in Vietnam. In 2014, China moved an offshore drilling rig into waters that Vietnam considers part of its exclusive economic zone, which prompted large demonstrationsand efforts by Vietnamto force the rig to move.

The two countries fought a brief but bloody border war in 1979, when China invaded Vietnam in an attempt to punish its neighbor for toppling the Khmer Rouge in Cambodia.